Capacitor banks are installed to improve the quality of an electrical supply by providing reactive power compensation and power factor correction in a power system. The use of capacitor banks has increased because they are relatively inexpensive, easy and quick to be installed, and can be deployed almost anywhere in a power system grid. Capacitor bank installations have other beneficial effects on the system such as improvement of the voltage profile, better voltage regulation, reduction of losses, increase of power transmission capacity and reduction or postponement of investments in the transmission and generation capacity.
A capacitor bank is assembled by a plurality of individual capacitor units. Each individual capacitor unit is a basic building block of the capacitor bank and includes a plurality of individual capacitor elements, arranged in parallel/series connected groups, within a metal enclosure. The units can be externally or internally fused, fuseless or unfused depending on the application of the bank. The elements can be connected to fuses and a group of elements is normally shunted by an internal discharge resistor in order to reduce the unit residual voltage after being disconnected from the power system. Each of the capacitor elements is constructed by winding two electrodes of aluminum foil separated by several layers of paper, or plastic film-insulated or a mixed dielectric of paper and plastic film.
Capacitor banks are normally further constructed using individual capacitor units connected in series and/or parallel to obtain a required voltage rating.
However, an internal failure in terms of operated fuses, failed elements and/or failed units in one or more quadrants may occur due to, for example the improper selection of the designed voltage rating that may result in continuous high voltage stress for the selected capacitor bank and eventually may lead to a dielectric failure of capacitor elements. Other causes for internal failure can be overcurrent, overvoltage, short-circuit, thermal failure and internal stress. It may also include maloperation of fuses due to bad fuse coordination.
The existing unbalance protection schemes are typically available to detect such an internal failure. For example, unbalance protection can be utilized in a variety of capacitor bank connections: grounded wye, ungrounded wye, delta, and single-phase. The H-bridge existing internal failure protection is based on a current measurement, typically by using a current transformer, in the link connecting two strings of capacitors together at the midpoints of the two strings. Any change in the capacitance of any capacitor will cause a change in the H current, also called unbalance current.
However, the existing unbalance protections based on the H-bridge scheme are liable to detect the number of internal failures incorrectly because an internal failure unbalance signal can be cancelled by having a combination of internal failures in any two or more adjacent quadrants. This consequently results in an ambiguity in terms of detecting the number of internal failures. Furthermore, such ambiguity may result in undetected failures in the capacitor bank, which may lead to a risk of fire or explosion accompanied by a severe damage of the whole capacitor bank before taking an early preventive action.
Furthermore, since a capacitor bank comprises a plurality of units each including a plurality of elements, failing to identify the location of an internal failure results in a costly maintenance because the whole capacitor bank has to be shut down and an exclusive search has to be carried out. More importantly, this could affect the availability of the capacitor bank.
U.S. Pat. No. 4,956,739 describes a protection system for a capacitor bank having a double-H bridge arrangement, where the unbalance currents are measured using two current transformers, and where failures are localized by phase angle calculations. However, no measure has been countered for a cancellation effect, which is a consequence of experiencing combination of internal failures in any two adjacent quadrants in H-bridge capacitor banks. The system therefore could not determine the exact number of internal failures in the bank, which affects the reliability and sensitivity of the system. Additionally, it is applicable only whenever there are two current transformers dividing each phase into 6 equal batteries of capacitor units with double H connected banks, not the typical H connected banks with one current transformer.
Document U.S. Pat. No. 4,219,856 (DANFORS ET AL) (D2) shows a protective device for a capacitive bank that counts current surges (see claim 1) and counting number of internal failures based on number of these surges, and includes polarity sensing means for distinguishing between surges emanating from one or the other of the capacitor branches (claim 6). Furthermore, D2 discloses (column 2, lines 25-30) the use of the stationary unbalance current protection relay disclosed in U.S. Pat. No. 3,143,687 (HJERTBERG ET AL) (D3). It would also be useful for these devices to provide a more reliable detection, especially in view of detecting combinations of internal failures with the possibilities of cancellation effects.
Therefore, a more sensitive and accurate internal failure detection and protection scheme for H-bridge arrangement is desired.